1. Field of the Invention
The present invention relates to an electrically programmable nonvolatile semiconductor memory device and, more particularly, to a flash memory which has a protection function of protecting each memory block from a rewrite or erase.
2. Description of the Related Art
A flash memory changes the threshold voltage by changing the charge amount of the floating gate of a memory cell transistor (erase/write operation) and accordingly stores data. For example, a negative threshold voltage is made to correspond to binary 1. A positive threshold voltage is made to correspond to binary 0.
Flash memories have been developed as ROMs in which data can be electrically rewritten. They are recently changing their position to a replacement for magnetic storage media represented by NAND flash memories. Examples of such memory devices are a flash memory card serving as a storage medium for a digital still camera or a flash memory serving as a storage element for user data in a cellular phone.
In line with progress in information technology, how to ensure security is attracting a great deal of attention. For example, a challenge in introducing a charging/payment system using a cellular phone is to prevent any alteration of the basic information of that system. In this case, partial data in the NAND flash memory must be prevented from being altered.
In the NAND flash memory, a read/write for each page (e.g., 528 bytes) and an erase for each block (constructed by a plurality of pages) are executed. In a system (e.g., a memory card) using such a NAND flash memory, file data is often managed for each memory block. For this reason, in many NAND flash memory systems, file information in each memory block is read after power-on. A file management table is created in a cache memory or the like, and then the NAND flash memory is controlled.
On the other hand, NOR flash memories still maintain the position of electrically programmable data ROMs. Some NOR flash memories have a protection function of protecting each memory block from a write or erase. However, as blocks to be protected are actually fixed, the NOR flash memories have no protection function as a replacement for magnetic storage media.
Jpn. Pat. Appln. KOKAI Publication No. 10-106275 discloses a ferroelectric memory which can set write inhibition/permission for a plurality of blocks. According to this prior art, since a ROM section and RAM section with write protection can freely be set, any setting error due to system runaway or the like can be prevented.
As described above, various proposals have been made for conventional nonvolatile semiconductor memory devices to ensure security in their memory structures. There are however no sufficiently effective measures in the chip size, operation speed, and convenience for users, and further improvement is demanded.